The operation of vehicles is prohibited if:

Explanation

In both cases; both the external lighting devices not working and a bus tyre tread of 0.8 mm.

External lights must work in exactly the mode defined for them: main and dipped beam, brake lights, indicators, tail lights and the number-plate lamp. The failure of any of these to work in the prescribed mode prohibits operation of the vehicle. Note that the criterion is not "coming on" but "working in the prescribed mode"; a brake light that comes on late, or an indicator that stays on instead of flashing, is equally a defect.

The reason is that lights are the vehicle's language with the rest of the road. All the calculations the driver behind makes about distance and braking are based on your brake light; if that light is out, he will not realise you are slowing until the moment of impact. The same logic applies to indicators and to lane-change and U-turn manoeuvres.

A tread of 0.8 mm is this question's numerical trap: 0.8 is the limit for a motorcycle, not a bus. A bus requires a minimum of 2.0 mm, so 0.8 is far below the legal limit and prohibited. A tyre with that tread depth is effectively bald, its longitudinal and lateral grip collapses on a wet surface, and in an emergency stop it sets the bus sliding.

• It is true that a faulty light prevents operation, but this is not the only fault raised in the question, and choosing it makes the answer incomplete. • A tread of 0.8 mm really is below the bus limit too, but the lighting fault is raised alongside it and you must accept both.

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